The goal of plant breeding is to combine various desirable traits in a single variety. Such desirable traits may include greater yield, resistance to diseases, insects or other pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, enhanced growth rate and improved shelf life.
The development of commercial lettuce cultivars or varieties requires the crossing of lettuce plants, and the evaluation of the crosses. Pedigree breeding and recurrent selection are examples of breeding methods are used to develop cultivars from breeding populations. Breeding programs combine desirable traits from two or more varieties or various broad-based sources into breeding pools from which cultivars are developed by selfing and selection of desired phenotypes. The new cultivars are crossed with other varieties and the inbred lines or hybrids from these crosses are evaluated to determine which have commercial potential.
All cultivated forms of lettuce belong to the highly polymorphic species Lactuca sativa that is grown for its edible head and leaves. Lactuca sativa is in the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, dandelion, artichoke and chrysanthemum. L. sativa is one of about 300 species in the genus Lactuca. There are many types of lettuce, and new types are constantly in development. Types of lettuce include Cutting/Leaf, Iceberg/Crisphead, Cos or Romaine, Batavian, Salinas Group, Latin, Butterhead, Great Lakes Group, Eastern (Ithaca) Group, Bibb, Vanguard Group, multileaf or Stem lettuce.
Fresh lettuce is available in the United States year-round although the greatest supply is from May through October. For planting purposes, the lettuce season is typically divided into three categories, early, mid and late, with the coastal areas planting from January to August, and the desert regions planting from August to December. Lettuce is consumed nearly exclusively as fresh, raw product and occasionally as a cooked vegetable.
Lifestyles change and the demand from restaurants and catering firms for colorful and interesting garnish for sandwiches and ready-to-use processed salads continue to rise. As a result, there is a demand for breeding companies to develop new varieties with specific shapes of leaves, specific average size of leaves, glossiness, prominent color and a wide variety of texture, as well as good yield.